I have one of those (or looks suspiciously like it) in my tank too. No damage seen on any of my corals (that looks crab related anyways, and I've had coral eating crabs so I can tell
). I caught him eating algae the other day, no opportunity to spear him or anything, ahem I mean catch him and put him in a more appropriate environment
I'm keeping an eye on him but so far I'm giving him the benefit of the doubt. However, if the opportunity arises he's a goner.